It seems wrong that the MBP should have a 256 GB SSD while the MBA has a 512 GB one, but I made that choice over 4 years ago. I swapped the 512 GB 5400rpm (slow) HDD for a much faster (at the time) Samsung 830 SD. As I was only using 25% of the capacity of the HDD I felt I could manage to use around 50% of a 256 GB SSD.
It's working out OK: still 117 GB free space.